I have a question about functional dependencies, instance contexts, and type inference. A specific example and question is in the attached code. In brief the question is: to what degree does type inference use the functional dependencies of an instance's class and context? I believe I am wishing it were more aggressive than it is. Please note that I have not enabled overlapping instances. Any suggestions regarding how to get the inferred type of |rite_t1| to be the one I anticipated would be much appreciated. Of course, I would also appreciate explanations of why I shouldn't anticipate it! The rest of this message is a copy of the attached code. Thanks, Nick I'm using GHC 6.6, but I see the same inferred types with 6.4.1.
{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} -- for the coverage condition
module FunDepEx where
A plain ole' isomorphism class.
class Iso a b | a -> b, b -> a where rite :: a -> b left :: b -> a
Isomorphism lifts through the sum bifunctor.
bifmap_either f g = either (Left . f) (Right . g)
instance ( Iso f f', Iso g g' ) => Iso (Either f g) (Either f' g') where rite = bifmap_either rite rite left = bifmap_either left left
Some types to play around with.
newtype MyChar = MyChar Char deriving (Show, Eq)
instance Iso MyChar Char where rite (MyChar c) = c left c = MyChar c instance Iso Char MyChar where rite c = MyChar c left (MyChar c) = c
My type inference confusion follows; the unit arguments are just to suppress the monomorphism restriction.
t1 :: Either Char a t1 = Left 'c'
rite_t1 () = rite t1
The inferred type for rite_t1 is rite_t1 :: (Iso (Either Char a) (Either f' g')) => () -> Either f' g' Why isn't the inferred type of rite_t1 the same as the ascribed type of rite_t1'?
rite_t1' :: Iso b b' => () -> Either MyChar b' rite_t1' () = rite t1